Net carbs are calculated by subtracting fiber and sugar alcohol carbohydrates from the total carb counts in food. This net figure is typically listed on foods that are marketed for low carb diets or diabetics. In order for carbohydrates to affect blood glucose levels, they must be broken down and digested in the small intestine. Fiber and sugar alcohols tend to pass through the small intestine without being digested, and thus have little or no impact on blood glucose levels.

A classic calculation of net impact carbs subtracts fiber from a food's total carbohydrates. For example, if a 25-carbohydrate snack contained five grams of fiber, the net impact carbs would be 20. This is done because naturally-occurring fiber is not commonly broken down in the small intestine. Also, sugar alcohols may have a lesser impact on blood glucose levels.

Sugar alcohols may be added to candies and dessert foods with low net carbs. These are synthetic sweetening agents that cannot be broken down during the digestive process. Three of the most common sugar alcohol ingredients are maltitol, sorbitol, and erythritol. Each has a different affect on blood glucose levels, with erythritol typically causing the least rise. Total carbohydrate counts generally include sugar alcohols, but need to be subtracted to calculate net carbs.

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These synthetic sweeteners are not typically digested and pass through the intestine. Eating foods high in sugar alcohols can lead to gastrointestinal discomfort, gas, and bloating. Other common side effects of consuming too much of these substances include diarrhea or loose stool.

Foods that list low net carbs typically assume lesser impact on blood glucose levels, based on scientific calculations and typical response in the small intestine. This does not mean every person will react in the same manner, however. Net carbs should only be used as a helpful guide to choosing foods with less glycemic load or glucose impact.

In the United States, some foods can list zero net carbs on the label even when carbohydrates are present. Powdered artificial sweetener packets, for instance, contain dextrose. This is a sugar and, therefore, a carbohydrate. One packet, or one serving, may contain less than one gram of carbohydrate, which is why zero net carbs can be listed on the food nutrition label. It can be important to remember that when more than one packet is consumed, total carbohydrates may add up to more than zero.

Low glycemic carbohydrates can be listed under several different names. Certain carbohydrates — like polydextrose, oligofructose, and inulin — are included in the total carbohydrate counts in food, but need to be subtracted in order to calculate net carbs. Scientific studies on the impact of these ingredients are minimal, but there is some evidence that the impact on blood glucose level is small.

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